Biofuels production for use as liquid motor fuels or for blending with conventional gasoline or diesel motor fuels is increasing worldwide. Such biofuels include, for example, ethanol and n-butanol. One of the major drivers for biofuels is their derivation from renewable resources by fermentation and bioprocess technology. Conventionally, biofuels are made from readily fermentable carbohydrates such as sugars and starches. Since these feed sources compete with the human food supply much recent work has focused on alternative feed sources for biofuels and other chemicals.
One alternate source of feeds is lignocellulosic feedstocks such as forest residues, trees from plantations, straws, grasses and other agricultural residues. However, the very heterogeneous nature of lignocellulosic materials that enables them to provide the mechanical support structure of the plants and trees makes them inherently recalcitrant to bioconversion. Also, these materials predominantly contain three separate classes of components as building blocks: cellulose (C6 sugar polymers), hemicellulose (various C5 and C6 sugar polymers), and lignin (aromatic and ether linked hetero polymers). Then other well-known technologies can convert the lignocellulosic biomass feed to syngas (also known as synthesis gas, primarily a mix of CO, H2 and CO2 with other components such as CH4, N2, NH3, H2S and other trace gases) This syngas is then fermented with anaerobic microorganisms to produce biofuels such as ethanol, n-butanol or chemicals such as acetic acid, butyric acid and the like. U.S. Pat. No. 7,285,402, the teachings of which are incorporated by reference herein, discloses methods for converting carbon monoxide, carbon dioxide, and hydrogen to acetic acid and ethanol by fermentation using anaerobic bacteria.
Anaerobic fermentations to produce biofuels and other chemicals can utilize any gaseous substrates that provide a carbon monoxide and/or carbon dioxide and hydrogen from a variety of sources. For example, US Patent Publication 2011/0300593 discloses sources such as steel mill off-gas as source of carbon monoxide and the teachings of which are incorporated by reference herein. Many other sources of substrates are available. For example, syngas can be made from many other carbonaceous feedstocks such as natural gas, reformed gas, peat, petroleum coke, coal, solid waste and land fill gas.
Ethanol can be produced from CO, CO2 and H2 using a variety of anaerobic bacteria, in particular such as those from the genus Clostridium. For example, various strains of bacteria that produce ethanol from gases are described and include Clostridium ljungdahlii, Clostridium autoethanogenum and Clostridium coskatii all of which are described further herein.
The production of ethanol and other products by the anaerobic microorganisms is influenced by many operating conditions within the fermentation zone. (see U.S. Pat. Nos. 5,173,429, 5,593,886, and 6,368,819, WO 98/00558 and WO 02/08438) Two primary conditions affecting the microorganism performance are the pH and oxidation-reduction potential (ORP) of the fermentation zone. WO2009/022925 discloses the effect of pH and ORP in the conversion of the gaseous substrates to products.
Microorganisms used in metabolic processes require nutrients and micronutrients and the particular supply of the nutrients can have profound effects on the growth and sustainability of the microorganisms. In fact these nutrients may be required to enable the microorganism to use carbon monoxide as its source of energy. For example, the microorganism may require the presence of metal co-factors for the metabolic functions of carbon monoxide dehydrogenase (CODH), and acetyl-CoA synthase (ACS). It is important that all of the required nutrients are provided in the proper amount and a bioavailable form.
Another of the required nutrients is a source of reduced sulfur, usually in the form of an organic sulfide such as cysteine. The cysteine provides a sulfur source necessary to support enzymatic processes occurring in a microbial culture. It is well known that microorganisms require sulfur in their enzymatic processes. In fact, the electron transfer mediator, ferredoxin, as well as Wood-Ljungdahl pathway enzymes acetyl-CoA synthase, and carbon monoxide dehydrogenase contain sulfur. Therefore, it is important to add sulfur in a bioavailable form and in sufficient supply to avoid inhibiting the growth or production of product by the microorganism.
As an alternative to cysteine, hydrogen sulfide has been found in many instances to be a source of the reduced sulfur needed for the metabolic processes of the microorganisms. Sulfur sources such as sulfide exist in equilibrium with hydrogen sulfide in typical fermentation media. Although hydrogen sulfide is less expensive than cysteine, it is toxic and thus requires special handling and is particularly dangerous in pure form. Supplying sulfur in the form of a sulfide salt such as sodium sulfide still results in a hydrogen sulfide concentration in the fermenter that may decrease over time due to evaporation. Moreover, hydrogen sulfide has a limited solubility in the media. Hydrogen sulfide may become highly volatile under the certain conditions that may be desired in the fermentation zone thereby exacerbating its use as a sulfur source. As a result, of the limited solubility of hydrogen sulfide in water the concentration of sulfur in solution can be significantly reduced by the conditions within fermentation medium. Accordingly, identification of improved or alternative sulfur sources for the microorganism in alcohol production required in fermentation systems using carbon monoxide or hydrogen and carbon dioxide gases as a feedstock would aid in achieving high alcohol production rates and low process operating costs.
Accordingly, the methods for producing ethanol or acetate from anaerobic fermentations would benefit from the discovery of sulfur compounds that can inexpensively provide the biological sulfur needs of the microorganisms under favorable fermentation conditions while also eliminating the disadvantages of hydrogen sulfide as a sulfur source.
US Patent Publication 20110300593 discloses the use of alternate sulfur sources. This document only specifically describes polysulfide, polysulfides, elemental sulfur and colloidal sulfur as the inorganic sulfur compounds for use as an alternative sulfur source. More importantly, the document describes the use of these specific sulfur compounds as means of providing hydrogen sulfide as the active species in the fermentation medium. As a result, this reference does not identify a sulfur additive that will provide a biologically available form of sulfur other than hydrogen sulfide and therefore does not overcome the high volatility problems of hydrogen sulfide.
Methods are sought to enhance the economics of syngas fermentation to produce C2 oxygenated organic compounds where the sulfur nutrient can be effectively and inexpensively supplied by the processes at an “as needed” rate.